Telomeres, the nucleoprotein structures that cap the ends of eukaryotic chromosomes, are composed of TTAGGG repetitive sequences that terminate in a 3'single-stranded G-rich overhang. Telomeres are bound by a complex array of proteins that help stabilize its formation and serve essential roles in preventing DNA damage checkpoint activation. Recent evidence suggests that telomere dysfunction is associated with increased human cancer incidence. Telomere dysfunction leads to the generation of dicentric chromosomes that initiate the breakage-fusion-bridge (BFB) cycle to elicit a cancer promoting genotype. We observed that conditional deletion of Pot1 (protection of telomere), a critical telomere binding protein, in mouse embryo fibroblasts (MEFs) resulted in telomere dysfunction, activation of DNA damage response (DDR), and p53-dependent replicative senescence in vitro. In addition, loss of Pot1 cooperated with p53 deficiency to initiate tumorigenesis in vivo. The objective of this research project is to generate a conditional Pot1 knockout mouse model to: 1) characterize the molecular determinants of Pot1-induced p53-dependent replicative senescence;2) to determine the biological function of p53 in loss of Pot1 initiated tumorigenesis in vivo;and 3) to cytogenetically characterize chromosomal aberrations in tumor samples, and investigate the function of these aberrations in human tumor initiation and progression. To achieve the objectives, Pot1-deficient mice will be crossed with mice bearing loss of cellular senescence (p21 knockout mouse) or apoptosis components (p53R172P knockin mice) to compare the latency of tumor development and tumor spectrum in mouse cohorts with dysfunctional telomeres. Finally, molecular cytogenetic techniques such as spectral karyotyping (SKY), array-comparative genomic hybridization (aCGH) and lentivirus-based siRNA will be used to identify chromosomal aberrations and validate potential molecular targets involved in tumorigenesis in conditionally deleted Pot1 mouse cohorts. The long-term goal of this study is to identify molecular targets involved in human cancer initiation or/and progression in the setting of telomere dysfunction.